Abstract 327: Utrophin Loss in the Absence of Dystrophin Induces Alterations in Cardiac-Related Protein Expression and Activity in the Heart and Muscle of Dystrophic Mice

Abstract

Duchenne Muscular Dystrophy (DMD) is an x-linked recessive disorder that affects 1:3500 males. The disease is characterized by the absence of the dystrophin protein, leading to progressive muscular degeneration in both skeletal and cardiac muscle. DMD is recapitulated in mouse models, the most severe model lacking both dystrophin and its compensatory homolog, utrophin (mdx:utro). This model mimics the onset of DMD much more rapidly than mdx. Nitric oxide (NO) synthesis is significantly diminished in DMD and mdx skeletal muscle. We have identified previously unreported functional differences in neuronal nitric oxide synthase (NOS1)-related nitric oxide production in cardiac musculature in mdx:utro mice, caused by the absence of utrophin. We have quantified mechanistic differences in L-Arginine mediated nitric oxide production via NOS1 in mdx and mdx:utro myocytes isolated via Langendorff perfusion. Interestingly, L-arginine transport levels were increased in the presence of this amino acid and occurred concurrently with increases in the number of mRNA transcripts for the low-affinity cationic amino acid transporter, CAT-2A. As CAT activity is an obligate process for L-Arginine entry to the cardiac myocytes, this may represent a potential compensatory mechanism related to decreased nitric oxide production. We have also found manifestations of ectopic cardiac-related protein expression in the quadriceps muscle of mdx:utro mice. In these tissues, we have found ectopic expression of the cardiac isoform of the SERCA2a calcium pump predominantly expressed in the heart. Additionally, we have found expression of slow-twitch Type 1 cardiac myosin heavy chain, indicative of a fiber type switch event. We have shown that expression of both of these proteins is co-localized to specific muscle fibers. Our analyses indicate that the appearance of these proteins is directly correlated with the lowered expression of the fast-twitch calcium pump (SERCA1) and muscle fiber types (Type 2a and Type 2b). These findings illustrate important molecular consequences of the absence of utrophin in mouse models of DMD. Identification of compensatory corrective targets may bear important therapeutic potential in muscular dystrophy and its associated cardiomyopathy.